In the related art, as an active layer (a semiconductor film) in a semiconductor device such as a field effect transistor, an inorganic-based semiconductor material represented by silicon is used.
However, the semiconductor device using the semiconductor film formed of the inorganic-based semiconductor material such as silicon has disadvantages as follows. Firstly, since a vacuum process, a high-temperature heat treatment, or the like is necessary, a large amount of energy is consumed. Secondly, since a high-temperature heat treatment is necessary, a type of substrate to be used is limited. Thirdly, investment in an expensive facility for fabrication is necessary. Fourthly, since the inorganic-based semiconductor material is hard and brittle, durability for bending or tensile stress is low.
In recent years, a semiconductor device including a semiconductor film formed of an organic semiconductor material has received attention. The semiconductor device has advantages as follows, as compared with a semiconductor device using a semiconductor film formed of an inorganic-based semiconductor material. Firstly, the semiconductor film can be formed at a low temperature. Secondly, since the organic semiconductor material is soluble in a solvent, the semiconductor film can be formed by coating. Thirdly, since the organic semiconductor material is soluble in a solvent, the semiconductor film can be formed by a printing method. Fourthly, since the semiconductor film can be easily formed by a coating or printing method, it has advantageous in terms of an increase in an area of the semiconductor film. Fifthly, since the semiconductor film can be formed at a low temperature, the semiconductor film can be formed on a flexible substrate which has low heat resistance, but is flexible, which is formed of a plastic, or the like, and thus a flexible semiconductor device can be fabricated. Sixthly, since characteristics of the semiconductor film can be controlled through substituent control of the organic semiconductor material, multiple functions and high performance of a semiconductor device can be attempted. Seventhly, low costs of a semiconductor device can be attempted.
Until now, as the organic semiconductor material suitable for coating or printing, pentacene derivatives, poly(alkyl thiophene)s, and the like have been used, and development in fabrication of a field effect transistor by a wet process using the organic semiconductor materials has been conducted. However, carrier mobility of the field effect transistor is equal to or less than 0.1 cm2V−1s−1, and is smaller than 1 cm2V−1s−1, which is mobility of a field effect transistor (a thin film transistor) using amorphous silicon of the related art.
Further, as compared with an inorganic semiconductor material, the organic semiconductor material has a problem related to carrier injection. That is, in general, it is said that in organic molecules, a molecule having a shallow HOMO (highest occupied molecular orbital) is unstable. In fact, in polyacene compounds, it is said that since anthracene having a short ring length has a deeper HOMO than pentacene having a long ring length, anthracene is stable. However, in a stable organic molecule having a deep HOMO, when a general metal is used as an electrode material, since a large energy difference between an HOMO of the organic molecule and a work function of the metal is occurred, effective carrier injection is expected to be inhibited by a Schottky barrier.
Further, the organic semiconductor material is unstable in the atmosphere or at a high temperature, as compared with the inorganic semiconductor material. That is, as described above, an organic molecule, which is capable of satisfactory performing carrier injection on an electrode, has a shallow HOMO, but the organic molecule is likely to be unstable in the atmosphere or at a high temperature. Further, it is known that the organic semiconductor material is decomposed through a reaction of its own material to oxygen. Based these reasons, it is known that characteristics of the semiconductor device using the semiconductor film formed of the organic semiconductor material are deteriorated.
Further, in a semiconductor device using a semiconductor film formed by spin-coating a solution in which an organic semiconductor material is dissolved in a solvent, it is difficult to secure characteristic uniformity in the plane. This is regarded to be caused by coating unevenness when the solution containing the organic semiconductor material is spin-coated.
In recent years, to solve the problems of the semiconductor device using the semiconductor film formed of the organic semiconductor material, by the inventors, it is suggested using a dioxaanthanthrene-based compound, such as 6,12-dioxaanthanthrene (also known as peri xanthenoxanthene, 6,12-dioxaanthanthrene (may be abbreviated as “PXX”)) as the organic semiconductor material (see Non-Patent Literature 1 and Patent Literature 1). When the dioxaanthanthrene-based compound is used, the above-described problems caused in the case in which the semiconductor film formed of the organic semiconductor material is used can be solved. For example, PXX is stable in the atmosphere and has excellent heat resistance (see Non-Patent Literature 1).